1. Field of the Invention
The present invention relates to an on-vehicle device mounted on a vehicle and used for a smart entry system or keyless entry system (hereinafter referred to as “system”) that communicates with a mobile device and remotely controls the locking/unlocking of a vehicle door or the like or the start of the engine, and more particularly to an on-vehicle device for determining a failure of a vehicle antenna.
2. Background Art
Recently, the system for locking or unlocking a vehicle door or starting an engine by remote control without using a machine key has become widespread in the field of the development of a vehicle such as an automobile. The system allows a user to automatically open or close the door lock of a vehicle by operating a desired push button of a mobile device, and allows an on-vehicle device mounted on the vehicle to detect the existence of the mobile device in the vehicle and to permit the start of the engine. Thanks to this system, the user does not need to insert the machine key into the keyhole for locking or unlocking, and the convenience is increased.
FIG. 3 is a block diagram of a conventional on-vehicle device. In FIG. 3, receiving antenna 20 receives a radio signal from mobile device 1, and on-vehicle device 29 transmits a converted electric signal to receiving circuit 21. Receiving circuit 21 feeds, into control unit 22, a received signal that has been returned to the state of the original transmitted signal supplied from mobile device 1 by amplifying or demodulating the electric signal. On receiving the received signal, control unit 22 compares the identification (ID) code of mobile device 1 included in the received signal with the code previously stored in control unit 22, thereby determining whether the received signal is correct or incorrect.
Control unit 22 drives driving circuit 23 using driving signal A, and supplies antenna current IA to vehicle antenna 27. Here, antenna current IA is supplied from power supply VD to driving circuit 23 through shunt resistor RS of detector 26, and flows to vehicle antenna 27 via driving circuit 23.
Detector 26 is formed by connecting differential amplifier 26A to both ends of shunt resistor RS, and differential amplifier 26A amplifies the voltage generated across shunt resistor RS by flowing of antenna current IA. Differential amplifier 26A feeds back detection signal B generated by amplification to control unit 22.
On receiving the radio signal from mobile device 1, on-vehicle device 29 collates the ID code of mobile device 1 included in the received signal with the code previously stored in on-vehicle device 29 using control unit 22. When the codes match, on-vehicle device 29 controls and makes a door actuator automatically open or close the door lock.
On-vehicle device 29 determines a failure of vehicle antenna 27. The reason for determining the failure is described hereinafter. There are a plurality of vehicle antennas 27, namely an internal antenna for the inside of the vehicle and an external antenna for the outside of the vehicle. In the description below, the external antenna disposed in a door knob is described as an example.
FIG. 4 is a schematic diagram of an essential component of the conventional on-vehicle device. In FIG. 4, each vehicle antenna 27 has antenna element 27A formed by interconnecting coil L and capacitor C in series, harness 27B formed by twisting two lead wires, and connector 27C connected to terminal T of on-vehicle device 29. On-vehicle device 29 is disposed at the back of the dashboard on the front side, and vehicle antenna 27 is disposed in a rear door knob on the rear side. The wiring distance between both of them, namely the entire length D of harness 27B, can exceed 6 m.
When vehicle antenna 27 is disposed in the door knob, harness 27B passes the hinge part of the door. Therefore, harness 27B can become caught between the door and the vehicle body by opening/closing of the door, or can be bitten during assembling the vehicle. A short circuit or opening failure can therefore occur disadvantageously.
This is the reason for determining the failure of vehicle antenna 27.
The operation of determining the failure of vehicle antenna 27 is described hereinafter.
First, control unit 22 feeds driving signal A into driving circuit 23, power supply VD is turned on based on driving signal A, and antenna current IA is supplied. Detector 26 converts antenna current IA into voltage with shunt resistor RS, and feeds back detection signal B to control unit 22. Here, detection signal B is generated by amplifying the voltage with differential amplifier 26A. As a result, control unit 22 determines the failure of vehicle antenna 27 based on detection signal B.
When vehicle antenna 27 is normal, the voltage of power supply VD becomes a remaining voltage after consumption by vehicle antenna 27, and the voltage of detection signal B becomes smaller than that of power supply VD. Control unit 22 determines that vehicle antenna 27 is normal.
When the voltage of detection signal B is 0 V, namely antenna current IA does not flow at all, control unit 22 determines “open failure” of vehicle antenna 27. When the voltage of detection signal B is equal to that of power supply VD, namely when entire antenna current IA is consumed in shunt resistor RS, control unit 22 determines “short circuit failure” of vehicle antenna 27.
Thus, conventional on-vehicle 29 determines the failure of vehicle antenna 27 using antenna current IA that flows in shunt resistor RS of detector 26, namely antenna current IA that is supplied to driving circuit 23. Here, shunt resistor RS is connected in series between power supply VD and driving circuit 23.
An example of the conventional art document information related to this technology is Japanese Patent Unexamined Publication No. S62-10704.
In conventional on-vehicle device 29, however, detector 26 is formed of shunt resistor RS and differential amplifier 26A, so that the circuitry becomes complex and expensive. When vehicle antenna 27 is normal, antenna current IA is consumed wastefully by shunt resistor RS.